It takes time. Usually guys can't retract it until about 16 or so.
You can get an erection and pull down on the shaft skin so that the head of the penis stretches the opening OR you can use your fingers to spread the opening.
Take your time, don't force it. Do it as often as you can. There may be some discomfort but you don't have to hurt yourself.
Yes, the head will be sensitive, that is NORMAL. It's been covered and protected and isn't used to being touched.